In a coating process using organic solvent type coating material, there are strong demands in recent years that complete separation and recovery of the volatile organic solvents be performed without discharging them to outside the process, i.e. out of the coating facilities, to prevent pollution, particularly because of the restriction on photochemical smog. However, in the coating process of a large size workpiece such as a car body, aqueous (water base; water-soluble) coating materials are now used, which contain solvents based on water instead of organic solvent coating materials because complete separation and recovery of solvents not only causes an economical burden and requires enlargement of large facilities but also gives no ultimate solution to the problem.
In the meantime, such aqueous coating materials cause some inconveniences in subsequent processes because water is evaporated much slower than organic solvents. Namely, when coating is performed on a wet-on-wet basis, dripping occurs, while, when the coated workpiece placed into an oven, bubbling occurs. Accordingly, it is necessary to provide a process (hereinafter referred to as "flash-off process") for evaporation to evaporate water to a certain extent before proceeding to the subsequent processes.
However, when this flash-off process is turned to spontaneous evaporation type, long time is required for flash-off because evaporation speed of water is slow. For this reason, the flash-off zone must be extended. This means that larger facilities are required.
In this respect, it has been proposed to shorten the flash-off time by positively heating the workpiece in the flash-off process.
As a heating apparatus for such purpose, a radiation (radiant) heating system using a (radiant) radiation heater is suitable for initial heating, as described in the Japanese Patent Publication No. 52-30170, because this system is free of the disadvantages of the convection heating system, such as suspended dust, and the disadvantages of the induction heating system, such as complicated facilities and the restriction on the workpieces, and because quick heating can be accomplished.
However, when this heating apparatus is adopted, a wide variety of workpieces are transported to the flash-off zone one after another. Then, some of the workpieces may be dried up, and the quality of coating may be decreased.